1. Field of the Invention
The invention relates to the art of making consumable foam type patterns for metal casting and, more particularly, to making dimensionally accurate polystyrene patterns from expanded polystyrene beads and the like.
2. Description of the Prior Art
A recently commercialized method of making metal castings is commonly called the evaporative casting process (ECP). In such process, a plastic foam pattern is embedded in a dry sand mold and molten metal is poured into the mold to evaporate and displace the pattern, the gases of evaporation migrating through the sand mold.
A commercialized method of making foam patterns for use in ECP usually involves (i) preparing beads (about the size of salt grains) of polystyrene and a thermal expanding agent such as pentane, (ii) heating the beads in a large bulk container to a temperature of about 215.degree. F. to preexpand such beads to form grains of a density of about 0.9-1.9 pounds per cubic foot, (iii) transferring the preexpanded pellets to fill a pattern mold equipped with cold water and steam plumbing, the pellets being heated for about one minute by steam at a temperature of about 215.degree. F. to expand, soften, distort and merge together to form a unitary foam pattern defined by the mold walls, and (iv) stopping the expansion process by running cold water through the mold and removing the pattern from the mold for storage at ambient conditions.
Such a mold pattern, during storage or post-curing, will shrink for several days (up to 28-30 days) in an amount of about 0.004-0.008 inch/inch. This is due to the slow outward migration of residual expanding agent (trapped in each bead) from the foam pattern, thus changing the pressure condition of the pattern structure and necessitating some contraction. This is disadvantageous because, in a high volume casting line, the unduly long storage of such patterns interrupts production; also, the application of a ceramic coating to such patterns, which is needed for the casting process, must be inordinately postponed.
Such polystyrene patterns have also been made by alternative methods, such as by heating the polystyrene beads to about 215.degree. F., the beads being prepared with an internal expansion agent. High pressure compressed air (about 10 pounds per square inch) is imposed upon the heating chamber to prevent expansion of the beads until a precise moment at which the collapse of such compressed air is brought about by the use of a vacuum to permit rapid expansion. There is insufficient time dwell to permit the extraction of trapped gases from the expanded beads and thus post-curing is necessary (see U.S. Pat. No. 3,452,124).
In another prior art technique, vacuum was used for a few seconds to allow for the expansion of the polystyrene beads--containing a foaming agent other than an expansion agent. The expansion was through a die to form solid strands or tubes of the polystyrene material. Again, the exposure of the vacuum, solely for purposes of expansion of the foaming-type beads, is insufficiently long to permit extraction of the trapped gases within the expanded foam. The vacuum has always been removed upon attainment of the shaped form, still requiring post-curing (see U.S. Pat. No. 3,822,331).
The common denominator for all of the above prior art modes is the practice of the curing (which must take place by way of shelf storage) at atmospheric conditions; this takes a minimum of seven days.
It would be helpful if some mode were devised to allow for rapid extraction of residual expanding agents without having the pattern out of the production line for any significant length of time.